The majority of aircraft utilize mechanical flight control systems that incorporate direct mechanical linkages to physically connect flight deck controls with the flight control surfaces through power control units. In normal operation, this type of flight control system provides the pilot with tactile and visual information regarding the position of the flight control surfaces through the orientation of the flight deck controls. Because most pilots are familiar with the use of such mechanical control systems, the system also provides the pilot with a certain level of psychological assurance that it is operating correctly. Tactile and visual feedback of system operation is provided even when the autopilot is engaged. Specifically, the autopilot system is interfaced with the flight deck controls through actuators that are connected in parallel with the system mechanical linkages. Thus, the autopilot actuators drive the flight control surfaces; and the column, wheel, and pedals of the flight deck controls are driven by the system's mechanical linkages in accordance with the flight control surfaces. A pilot, in tactile and visual communication with the controls, therefore maintains an awareness of autopilot activity and is reassured of appropriate behavior, or alerted when the behavior of the autopilot appears inappropriate.
An alternative flight control system does not include direct mechanical linkages between the pilot's controls and the aircraft control surfaces. Instead, in this system, the pilot's commands, input through flight deck controls, are converted to electrical signals, which are then electronically processed to produce commands for control units that in turn appropriately orient flight control surfaces. In this system, there is nothing to drive the flight deck controls when the autopilot is operating. Thus, in this electronically driven flight control system, also known as a "fly-by-wire" flight control system, the pilot receives no tactile or visual feedback through column, wheel, and pedal flight deck controls that provide information regarding the operation of the autopilot. Instead, these flight deck controls are stationary while the autopilot is engaged. Without the visual and tactile information provided through feedback via the flight deck controls, the pilot cannot maintain awareness of autopilot activity. Consequently, the pilot may not be aware of inappropriate autopilot activity and may not be as aware of the orientation of aircraft control surfaces. These situations are generally unacceptable to pilots. Also, when the pilot elects to disengage the autopilot, the flight deck controls may be in their neutral or "rest" positions while the autopilot may have set corresponding flight control surfaces at different positions, not corresponding to the flight deck controls. Thus, on disengagement of the autopilot, there may be an abrupt adjustment between the positions of flight deck controls and flight control surfaces to bring these into correspondence, potentially causing passenger discomfort and alarm.
There exists a need for a system that provides a pilot with an awareness of autopilot activity in a fly-by-wire flight control system, especially one that utilizes traditional column, wheel, and pedal flight deck controls. The system should be safe, reliable, compatible with, and readily integrated into, the fly-by-wire control system. Desirably, the system should prevent the possibility of abrupt changes in orientation of aircraft flight control surfaces on disengagement of the autopilot.